The present invention relates to a hammer drill. More particularly, it relates to such a hammer drill which has a tool, a tool holder and drive means for selectively imparting torque and/or axial impact to the tool.
Hammer drills of this type have already been proposed in the art. Such hammer drills comprise a tool which has a shaft provided with two grooves of a semi-circular cross-section extending lengthwise an axis of the shaft and closed at both their axial ends. The closed end portions of the grooves is of a spherical form. Radially movable ball-shaped locking elements engage the above-mentioned grooves. Torque imparted by the drive means to the tool holder is transmitted to the tool shaft by means of these locking elements. An intermediate impact element transmits impacts to a trailing end portion of the tool shaft. However, the spherical end portions of the grooves serve at the same time as axial stop means for limiting axial movement of a drill spindle in a recess of the tool during transmission of axial impact and thereby also take part in transmission of the latter. In this construction the ball-shaped locking elements serve for simultaneously transmitting torque and axial locking, and therefore are subject to very high wear. This also does not create conditions for direct low-loss and recoil-less transmission of impact energy. In addition, this results in that a tool holder is heated during operation to a substantial degree, and also in that relatively high pressing force is required.